1. Field of the Invention
The present invention relates to an active vibration insulator, especially, an active vibration insulator for actively inhibiting the vibrations of vibration generating sources, such as on-vehicle engines, from transmitting.
2. Description of the Related Art
Active vibration insulators of this type drive an active engine mount, which can generate active vibrating forces, and can thereby inhibit vibrations, which are transmitted from an engine, from transmitting to the other parts of a vehicle. However, when the active engine mount is thus driven, it consumes energy so that the vehicle's mileage has deteriorated. Hence, Japanese Unexamined Patent Publication (KOKAI) No. 10-318, 325, for instance, discloses to relieve the mileage deterioration accompanied by actuating an active engine mount. The disclosed technique utilizes the fact that, when the levels of vibrations, which are transmitted from parts of a vehicle other than an engine, are high, the levels of vibrations, which vehicle passengers sense, are not reduced so much even if an engine mount is driven to reduce the levels of vibrations, which are transmitted from the engine. Specifically, when the levels of vibrations, which are transmitted from parts of a vehicle other than an engine, are high, signals for actuating an active engine mount are kept low. By thus keeping the actuating signals low, it is possible to reduce the energy, which the active engine mount consumes. As a result, it is possible to relieve the mileage degradation.
However, when a vehicle travels on bad roads, the vehicle vibrates greatly. As the vehicle vibrates, the plunger of an electromagnetic actuator, which an active engine mount of an active vibration insulator carries, reciprocates regardless of control. Note herein that, when the plunger of an electromagnetic actuator reciprocates, the inductance of the electromagnetic actuator changes because the distance between the plunger and core of the electromagnetic actuator varies. Moreover, when actuating the active engine mount, there is a fear that an overcurrent might flow in the core of the electromagnetic actuator and the actuating circuit of a controller depending on the changing inductance, because an actuating voltage is applied to the coil, which makes the core of the electromagnetic actuator. This fact results in a fear that, even when the actuating signals for the electromagnetic actuator of the active engine mount are kept low during the vehicle's driving on bad roads as described above, overcurrents might flow in the core of the electromagnetic actuator and the actuating circuit of a controller. In addition, when an overcurrent flows in the coil and actuating circuit, there occurs a fear that the electromagnetic actuator and controller might break down.